1. Field of the Invention
This invention pertains to a method for forming a pattern of functional material on a substrate, and in particular, the method uses an elastomeric stamp having a relief surface to form a pattern of open area on the substrate where the functional material is applied.
2. Description of Related Art
Nearly all electronic and optical devices require patterning. Microelectronic devices have long been prepared by photolithographic processes to form the necessary patterns. According to this technique a thin film of conducting, insulating, or semiconducting material is deposited on a substrate and a negative or positive photoresist is coated onto the exposed surface of the material. The resist is then irradiated in a predetermined pattern, and irradiated or non-irradiated portions of the resist are washed from the surface to produce a predetermined pattern of resist on the surface. To form a pattern of a conducting metal material, the metal material that is not covered by the predetermined resist pattern is then etched or removed. The resist pattern is then removed to obtain the pattern of metal material. Photolithography, however, is a complex, multi-step process that is too costly for the printing of plastic electronics.
Microcontact printing is a flexible, non-lithographic method for forming patterned materials. Microcontact printing potentially provides a significant advance over conventional photolithographic techniques since the contact printing can form relatively high resolution patterns on plastic electronics for electronic parts assembly. Microcontact printing can be characterized as a high resolution technique that enables patterns of micron dimensions to be imparted onto a substrate surface. Microcontact printing is also more economical than photolithography systems since it is procedurally less complex, not requiring spin coating equipment or a clean room environment. In addition, microcontact printing potentially lends itself to reel-to-reel electronic parts assembly operations that allows for high throughput production than other techniques, such as photolithography and e-beam lithography (which is a conventional technique employed where resolution on the order of 10 s of nanometer is desired). Multiple images can be printed from a single stamp in reel-to-reel assembly operations using microcontact printing.
Microcontact printing technique is a possible replacement to photolithography in the fabrication of microelectronic devices, such as radio frequency tags (RFID), sensors, and memory and backpanel displays. The capability of microcontact printing to transfer self-assembled monolayers (SAM) forming molecular species to a substrate has also found application in patterned electroless deposition of metals. SAM printing is capable of creating high resolution patterns, but is generally limited to forming metal patterns of gold or silver with thiol chemistry. Although there are variations, in SAM printing a positive relief pattern provided on an elastomeric stamp is inked onto a substrate. The relief pattern of the elastomeric stamp, which is typically made of polydimethylsiloxane (PDMS), is inked with a thiol material. Typically the thiol material is an alkane thiol material. The substrate is blanket coated with a thin metal film of gold or silver, and then the gold-coated substrate is contacted with the stamp. Upon contact of the relief pattern of the stamp with the metal film, a monolayer of the thiol material having the desired microcircuit pattern is transferred to the metal film. Alkane thiols form an ordered monolayer on metal by a self-assembly process, which results in the SAM being tightly packed and well adhered to the metal. As such, the SAM acts as an etch resist when the inked substrate is then immersed in a metal etching solution and all but the SAM protected metal areas are etched away to the underlying substrate. The SAM is then stripped away leaving the metal in the desired pattern.
A method of transferring a material to a substrate, particularly for light emitting devices, is disclosed by Coe-Sullivan et al. in WO 2006/047215. The method includes selectively depositing the material on a surface of a stamp applicator and contacting the surface of the stamp applicator to the substrate. The stamp applicator may be textured, that is have a surface with a pattern of elevations and depressions, or may be featureless, that is, having no elevations or depressions. The material is a nanomaterial ink, which includes semiconductor nanocrystals. Direct contact printing of the material on the substrate eliminates the steps associated with printing of SAM in which excess material that does not form the desired microcircuitry pattern from the substrate is etched away or removed.
Direct microcontact printing of SAM of thiol materials or other materials such as those described in WO 2006/047215 may be achievable in microelectronic devices and components having a high density of features. However, microcontact printing of devices and components having the pattern of fine resolution lines of functional material separated by relatively large featureless areas where no functional material resides can be problematic. The stamp can sag in areas between features where the density of features is low or the separation of between features is large. Sagging of the relief surface of the stamp is a phenomenon in which a lowermost surface of recessed areas of the relief structure collapse or sag toward an uppermost surface of the raise areas. Sagging may also be called roof collapse of the stamp. Sagging of the relief surface can cause the recessed areas to print material where there should be no material. The recessed areas sag into contact sufficient to transfer material onto the substrate in undesired regions that are not a part of the pattern of lines being formed. Sagging of the recessed areas of the stamp can even be exacerbated when pressure is applied to the stamp. Pressure on the stamp is sometimes necessary in order to achieve transfer of the material pattern to the substrate. If the material transferred is large, it may contact one or more of the pattern lines of the functional material, which can destroy the use of the component. Microcontact printing of conductive patterns, particularly using SAM layers, where the sagging of the stamp transfers material onto background areas can lead to shorting the devices or components.
In addition to the feature density pattern incorporated in the stamp, the elastic nature of stamp may contribute to sagging in featureless areas. The stamp used for microcontact printing is elastomeric in order for the stamp to sufficiently contact the substrate while conforming to various surfaces including on cylindrical or spherical surfaces, or discontinuous or multiplanar surfaces. However the features of the stamp may have an aspect ratio (determined by the width of features divided by height of features on the stamp) such that sagging is caused in the recessed areas between the pattern of fine resolution line features.
So it is desirable to provide a method for forming a pattern of a functional material, such as a conductor, semiconductor, or dielectric material, onto a substrate. It is also desirable for such method to have the ease of microcontact printing with an elastomeric stamp, but not be limited to printing onto metals. It is also desirable for such a method to avoid the problem of transfer of the functional material in featureless areas of the pattern.